Epidural anesthesia blocks pain sensation at nerve roots that branch directly from the spinal cord by bathing them with local anesthetic agents delivered to the epidural space, a small space adjacent to the outer protective covering of the spinal cord. This route of anesthetic delivery provides an effective method for pain control during childbirth, major surgery, and chronic back pain. However, accessing the epidural space to administer anesthetic remains challenging due to its small size and proximity to the spinal cord. The currently accepted method of blindly accessing the epidural space with a straight needle is often a time consuming process of trial and error that carries a complication rate of 2-20%. The excessive time demands of placement and threat of complications result in hesitation and underutilization of epidural anesthesia. Less than half of the 7 million obstetric and surgical patients eligible for epidural anesthesia receive it.
Epidural anesthesia is a block on pain sensation at the location of the nerve roots which exit bilaterally from the spinal cord at each vertebral level. Once the needle or a small catheter is positioned appropriately, local anesthetic such as lidocaine is injected into the epidural space to bathe the spinal nerve roots, resulting in loss of pain sensation. Epidural anesthesia has been demonstrated to reduce stress response to surgery, to decrease intraoperative blood loss, to lower postoperative incidence of thromboembolic events, and to decrease morbidity and mortality in high-risk surgical patients. (See Bernards C M “Epidural and Spinal Anesthesia”. Clinical Anesthesia, 5th Edition. Ed. Barash P G, Cullen B F, Stoelting R K. Philadelphia: Lippincott Williams & Wilkins, 2006). In addition, a catheter can be left in the epidural space for up to 5 days to provide continuous pain management in the postoperative setting, where epidural anesthesia has been demonstrated to be more effective in enabling rapid patient mobilization and earlier return of digestive function than other pathways for administering pain medications. (See Chandraskhar S and Pian-Smith M C. “Spinal, Epidural, and Caudal Anesthesia” Clinical Anesthesia Procedures of the Massachusetts General Hospital, 6th Edition. Ed: Hurford W E, Bailin M T, Davison J K, Haspel K L, Rosow C, Vassallo S A; Department of Anesthesia and Critical Care, Massachusetts General Hospital. Philadelphia: Lippincott Williams & Wilkins, 2002).
Accessing the epidural space can be extremely challenging. The epidural space is a potential space that is generally collapsed and enlarges when the tissues that bound it are separated. FIG. 1B illustrates the tissues that define the epidural space 19 including the dura mater (or dura 5) which is a protective covering that sheaths the spinal cord 4, the ligamentum flavum 6 which is a ligament adjacent to the dura 5 that runs longitudinally along the spinal column, and the bony sides of the vertebral canal. Other anatomical structures near the epidural space 19 illustrated in FIG. 1A include the pedicle 11, vertebral body 1, intervertebral disc 2, transverse process 10, spinous process 9 and a spinal nerve root 3. To access the epidural space 19, the patient is positioned either seated or on their side and instructed to flex their back outward to maximize spacing between the outer vertebral components. The spinous processes are palpated, and the interlaminar space is estimated. A needle trajectory is then chosen by the anesthesiologist and a Tuohy needle is inserted in the midline. This needle has both (1) a cutting tip which is offset in order to reduce inadvertent injury to adjacent structures and (2) a hollow lumen to allow for placement of a small catheter through which pain medication can be administered. As the needle is advanced, it passes through (in order from the skin 8): soft tissue 14, interspinous ligament 7, and ligamentum flavum 6 then ideally stops in the epidural space 19.
FIGS. 1A and 1B illustrate conventional placement of the needle 15 into the epidural space 19. FIG. 1A is a perspective view of the needle 15 in position with the surrounding anatomy. FIG. 1B is a section view the epidural space 19 showing a properly placed needle 15 and the creation of the epidural space by injecting a fluid 12 from the syringe 20 that is rigidly connected to the needle 15.
Prior to encountering the ligamentum flavum, a specially designed glass or plastic low-resistance syringe 20 filled with air or saline 12 is attached to the Tuohy needle 15. The needle 15 then is advanced slowly and gentle pressure is maintained on the syringe plunger 18 to assess the resistance to flow at the tip 16 of the needle 15. A loss of resistance to flow, as assessed through subjective feel when the air or fluid 12 is ejected from the syringe 20, indicates that the needle 15 has passed through the ligamentum flavum 6 into the epidural space 19. The needle 15 is held in position carefully to allow placement of the epidural catheter 25 then withdrawn from the epidural space over the catheter 25. Conditions such as degenerative joint disease of the spine and morbid obesity add to the difficulty of epidural access.
The challenges of accessing the epidural space can lead to complications in 2-20% of patients. The most commonly reported complications in the literature are headaches due to puncture of the dura, failure of pain blockade, backache, and epidural vein puncture. FIG. 1C illustrates a section view of the epidural space 19 with a puncture 22 in the dura 5 produced by the needle distal tip 16. FIG. 1D illustrates a section view of the epidural space 19 with an epidural vein 24 ruptured by the needle distal tip 16. FIG. 1E is a section view of the epidural space 19 with a catheter 25 improperly deployed outside the epidural space 6.
Postdural puncture headache (PDPH) is estimated to occur in 1-5% of all epidural procedures. The headache results from leakage of cerebrospinal fluid (CSF) 13 through an accidental dural puncture by the epidural needle. Initial treatment is bed rest requiring hospitalization, and in a significant number of patients with PDPH, an injection of blood into the epidural space, known as a blood patch, is required to close the inadvertent puncture site. Failure of effective pain control occurs in 5-20% of patients with 10-15% of these failures attributed to incorrect epidural catheter placement, which then results in epidural replacement or reliance on less effective means of pain control. Postoperative backache occurs in up to 30% of patients and can lead to temporary disability. Inadvertent puncture of a vein adjacent to the dura occurs in 1-11% of epidural procedures. If recognized, this is a minor complication requiring a new puncture; however, if unrecognized, catheter placement in an epidural vein can result in toxic systemic administration of anesthetic. Additional complications including significant nerve damage, meningitis, paraplegia, and death are rare (1 in 10,000 to 1 in 100,000).
The current technique of epidural access involves advancement of a Tuohy needle into the epidural space. This method relies heavily on a steady hand and the ability to immediately halt needle advancement once loss of resistance is detected to avoid damaging critical structures including the dura. Despite proven patient benefits, many practitioners are reluctant to use epidural anesthesia because of the challenges and risks described above. A survey of local practitioners revealed that excess time and fear of complications are factors that significantly limit the utilization of epidural anesthesia. What is needed are improved devices and methods for accessing the epidural space.